1. Field of the Invention
The invention pertains to a step aging process for aluminum silicon alloy castings capable of increasing both the tensile strength and the elongation of the casting. Complex castings having both thin and thick sections may be heat treated without excessively averaging the thin sections.
2. Description of the Related Art
Aluminum silicon alloy castings are produced in high volume for diverse applications. In many of these applications, for example cylinder blocks and heads, transmission castings, and the like, the castings may be quite complex, and more often than not, have portions of the casting with thick sections, for example crankshaft webs, while other portions have thin sections. To obtain adequate physical properties such as tensile strength, elongation, and hardness, aluminum silicon castings are generally subjected to a heat treatment.
The most popular Al—Si casting alloys (e.g. 319, 356, 390) are strengthened through the mechanism described as age hardening or precipitation strengthening. The process usually consists of three steps; first, the alloying elements are dissolved into the aluminum solid solution at an elevated temperature. This step is called the solution treatment and is usually performed as a separate operation from the casting process. After solidification, the casting is removed from the mold and then placed in a separate furnace to be reheated to a temperature just below the solidus and held for a period of time sufficient to dissolve precipitates and saturate the ∀ aluminum phase with solute atoms (usually Cu and/or Mg). In addition, some speroidization of the insoluble particles (such as silicon) will accompany “solutionizing.”
Following solutionizing, the casting is rapidly cooled during the second step of the precipitation strengthening process, termed “quenching.” The quench must be rapid enough to restrict diffusion and prevent the solute atoms from precipitating out of solution. A requirement of effective solute elements is that the maximum solubility in aluminum must increase with temperature, so that when the temperature is rapidly lowered, the aluminum will contain more than the equilibrium solute content and become “super-saturated.” The super-saturated state is a non-equilibrium state. Since the super-saturated aluminum composition contains more than ten times less solute atoms than the precipitate, solute atoms must cluster together to form regions of higher solute concentration and leave other areas of reduced solute concentration before a precipitate can form.
The difference between the equilibrium solute concentration in solution at the solution temperature and the equilibrium solute concentration in solution at the aging temperature provides the driving force for the precipitation reaction. The lower the aging temperature, the higher is this difference and therefore the higher is the driving force. Conversely, the lower the temperature, the lower is the atomic mobility.
Thus, the precipitation reaction is governed by the trade-off between the compositional driving force against the temperature-controlled atomic mobility. Some precipitation occurs even at room temperature. At low temperature the compositional driving force is high, but since the atomic mobility is low, the diffusion of solute atoms is slow and therefore the precipitation reaction is sluggish. At higher temperatures, the atomic movement is amplified making cluster-formation more rapid, but the compositional driving force is lower, resulting in a lower quantity of precipitate forming.
The choice of aging temperature in conventional heat treatment is a trade-off between reaction rate and the total amount of precipitate formed. The hardness and strength of the component is strongly controlled by the amount of precipitate formed during aging, the casting is reheated to an intermediate temperature to nucleate the strengthening precipitates. The precipitation reaction itself is a multi-step process, causing the strength and hardness of the casting to rise with time and temperature through some peak hardness value, and then decrease again. When the aging temperature is increased, peak hardness is obtained in a shorter time, but at some expense to the level of peak hardness. Thus, there is an optimal combination of temperature and time resulting in an optimum compromise between peak strength and process time constraints.
Control of each of the above steps is vitally important to achieving the combination of strength and ductility for the particular service application. Some castings are purposely aged at higher temperatures or for longer times to obtain a condition past peak hardness. This “overaged” condition exhibits a lower tensile strength than the peak aged condition, but the increase in tensile elongation (damage tolerance) and dimensional stability can be more important than strength in many applications.
The precipitation reaction involves a diffusion-controlled agglomeration of atom clusters to form zones rich in solute. At a later stage, a discrete phase precipitates from this zone. This clustering and precipitation causes strength to increase by the increase in localized lattice strain. Still later, the precipitates grow in size until the total system energy can be decreased by formation of an interface. At this point, the particle becomes an incoherent phase and the lattice stain decreases significantly with an accompanying drop-off in hardness and tensile strength. The precipitation of the particles is also accompanied by changes to the physical dimensions of the casting with time at temperature. Therefore, for applications with critical dimensional tolerances, the casting is heat treated past the peak hardness to the point where most of the dimensional change has occurred and then it is machined to the required dimensions.
The heat treatment of aluminum castings is an energy- and capital-intensive process that can involve up to 2 days or longer of in-process part heat treating at any given time. In addition, because of significant differences in casting microstructure from location to location within the part, the properties, both as-cast and after heat treatment, will vary with location within the part. Thus, microstructure and heat treatment are currently optimized for properties in a given location within the casting. The remainder of the casting may have inferior properties.
In addition, conventional heat treatment results in differential temperature ramps to the solution and aging temperatures due to part geometry driven by relatively poor heat transfer from the furnace atmosphere to the part. This results in different parts of the casting effectively receiving different heat treatments. The quenching operation suffers similar restrictions, although in a compressed time window. However, the reduced time differential still results in severe stress-induced distortion and even cracking resulting from differential cooling.
To compound these difficulties, the thin casting sections that naturally contain the finest microstructure due to the more rapid solidification are exactly the same locations that heat and cool the fastest during heat treatment, for the same reason; more favorable heat transfer geometry. This causes the longest time at temperature in the locations with the shortest diffusion distances as well as the greatest amount of solute already in solution, exactly the opposite of what is desired. Thus, in order to get the desired condition in heavier sections of a casting, other locations will become excessively overaged. However, this is usually partially offset by a significant improvement in properties caused by the refined microstructure caused by more rapid solidification in aluminum alloys. A refined microstructure is beneficial in that it usually causes a reduction in flaw size such as porosity and inclusions. This is independent of heat treatment.
The problems involved with prior art aging processes can be described by referring to FIG. 1. FIG. 1 is a chart of time versus temperature with several temperature regimes highlighted. The horizontal lines in the figure represent physical characteristics of the alloy comprising the casting, which vary depending upon the alloy composition. These are thermodynamic quantities and are independent of microstructural fineness. The Liquidus is the temperature at which solidification begins and the Solidus is the temperature at which solidification is complete. The Solvus line is the temperature above which the solute is entirely in solution; below this the alloy can exist as a two-phase mixture. Therefore, solution treatment is performed at a temperature between the Solidus and the Solvus. The group of horizontal lines between 100 and 200° C. represent various stages of the precipitation reaction. This is the aging regime. For temperatures above the Solvus, the precipitates are dissolving and for temperatures below this line, they are growing and coalescing.
During solution treatment, section 1, a relatively thin section of the casting heats up rapidly to the solution temperature. Section 2, however, is much thicker and takes much longer to reach the solution temperature. Likewise, upon cooling section 2 lags section 1.
The heavy structure of section 2 has relatively large diffusion distances compared to section 1. Section 2 also remains at the solution temperature for a shorter span of time than section 1 due to the time lag to reach that temperature. In addition, section 2 also spends a significantly greater amount of time in the temperature zone below the Solvus, where the precipitates and secondary eutectic phase particles are coarsening. Therefore, as the casting is heating to the solution temperature, the heavy section 2 undergoes farther coarsening even as the precipitates that formed during casting solidification are being dissolved in the thin section 1. Thus, section 2 would require an even longer time at the solution temperature to completely dissolve the precipitate. Since the heavy casting sections also exhibit greater solute coring in the aluminum phase, more time is needed for diffusion to eliminate these concentration gradients.
Upon quenching, section 2, again spends more time in the precipitate growth region, leading to less supersaturation and thus less strengthening potential. However, it is usually just this heavy section that will bear the greatest stresses in the final application, so the process has to be optimized for the properties in this section.
Heating to the aging temperature results in heating rate distributions following the same general patterns as described for the solutionizing treatment. The consequence, however, of the pattern of the heating rate difference is very different metallurgically.
Since the precipitation process is driven by the balancing of compositional driving force against the atomic mobility, and each are affected by the temperature in the opposite direction, it can easily be seen that precipitation will vary throughout the part depending on the differences in temperature. The greater the difference, the larger the variation and thus, the greater is the variation of properties throughout the casting.
As the precipitates begin to form, the hardness and strength increase with time at temperature and the ductility decreases due to an increase in lattice strain energy created by the atomic spacing mismatch between the precipitate and the matrix.
As the precipitates grow, the local strain at the precipitate-matrix interface increases until it reaches a maximum at which the system energy can be reduced by breaking the bonds between the precipitate and the matrix, forming a phase boundary. As more precipitates become separated from the matrix by these boundaries (decoherent with the matrix), the mismatch stress are relieved thereby decreasing the hardness and strength and increasing the ductility. Thus, the common observation is that for a given microstructure, the hardness and strength vary inversely with the ductility.